Semiconductor wafers are annealed during fabrication for a variety of reasons. For example, wafers are annealed to electrically activate dopant atoms in a semiconductor lattice. The thermal energy applied via the anneal process causes the atoms to diffuse into substitutional positions in the semiconductor lattice. Anneals are also used to change film-to-film or film-to-wafer substrate interfaces, densify deposited films, change states of grown films (e.g., epitaxial regrowth), repair damage from ion implantation, or drive interdiffusion of atoms from one material layer into another material layer.
Furnaces or ovens are often used to apply the thermal energy to the semiconductor wafers. Unfortunately, complications arise during high temperature anneals due to prolonged or uneven high temperature heating. Significant diffusion of dopant and other atoms can occur.
Rapid thermal processing (RTP) methods have been used to minimize undesired diffusion of dopants during heating. Common RTP methods are lamp and laser annealing. Such RTP equipment is used to heat a wafer to high temperatures on a short timescale, e.g., several seconds.
Although both lamp and laser heating can achieve the temperatures needed to repair a disordered silicon lattice and activate dopant atoms, both techniques may create uneven heating. For example, the uneven heating may arise from emissivity differences in near surface device materials. Moreover, photons used in lamp and laser heating cannot penetrate past the near surface region of the semiconductor wafer.
Microwave heating has been presented as a possible alternative to conventional RTP methods. Microwave heating of silicon may allow for more even, volumetric heating of the wafer due to the greater penetration depth associated with microwave radiation. In one approach, a microwave susceptor wafer is used when the target wafer does not absorb microwave energy sufficient for dopant activation or epitaxial regrowth. A microwave susceptor wafer, such as a silicon carbine (SiC) wafer, is exposed to the incident microwave radiation and placed sufficiently near the target wafer for conductive heating. The SiC wafer typically acts as a platform on which the target wafer is placed.